1. The Field of the Invention
This present invention relates to communication networks, and more specifically to methods and apparatus for stabilization of photonic data in order to narrow bandwidth requirements for channels in multiplexed or other transmission systems.
2. Background
Legacy sources of photonic signals are typically lasers, light emitting diodes, microwave transmitters, and the like. Traditionally, legacy photonic systems suffer from various limitations on the precision of the characteristic parameters for a given signal. For example, lasers often produce a comparatively broad spectral output of a light signal. In certain circumstances, lasers or other photonic sources may drift from one frequency to another over a comparatively broad range of frequencies.
Often, since light is electromagnetic radiation dependent upon the theories of quantum mechanics, the selection of a frequency of emission is actually a quantum event. Accordingly, frequencies may actually hop. Frequency hopping in a photonic source may also be a direct result of certain geometries or chemistries that produce substantially equivalent probability, desirability, or physical possibility for generation of signals at multiple frequencies. Accordingly, frequency hopping may exist, causing a requirement to observe, track, accommodate, or assign a comparatively large bandwidth to each signal or channel being relied upon.
Typically, a signal does not contain energy at a single frequency. A modulated signal may include several frequencies. Often, legacy photonic sources have comparatively large deviations from a main frequency intended, desired, or nominally rated for a particular device.
Wavelength stabilization or wavelength shifting is needed. However, according to technical experts writing in the photonic industry, semiconductor laser diodes exhibiting multi-mode behavior are not considered suitable for applications requiring extended distance of transmission, or for applications requiring wavelength (frequency) multiplexing. Moreover, some writers characterize attempts at wavelength conversion as being laboratory curiosities, having no practical implementations known in commercial products or systems.
The result of the variation in the actual spectral output of a legacy photonic source, when compared to the desired or nominal value, is excessive use of available wavelength (frequency) ranges (bandwidth) required to be allocated to a particular channel or line of data. In order to improve the situation, either more equipment is required, or replacement of old equipment with newer more precise equipment is required. Both options amount to expense, substantial expense.
Accordingly, telecommunication systems can become bandwidth limited. Moreover, typically, the actual photonic transmission medium (e.g. light fiber, etc.) can carry substantially more information than the equipment attached to each end can send or receive. Thus, the capacity of conventional fiber transmission systems could be substantially improved if the signal generation, signal management, multiplexing, de-multiplexing, detection, etc. equipment could be improved to operate within a narrower, more reliable range (bandwidth) of wavelength of frequencies.
One benefit to using the current carrier medium with a more finely subdivided date bandwidth is the substantial increase of useable information bandwidth. The alternative, is to lay more cable, (fibers) in order to support more end equipment for sending and receiving signals.
Several difficulties arise from the incompatibility of receivers with either the carrier medium, or a legacy photonic source. For example, a legacy photonic source is extremely expensive to replace. Thus, a more modern receiving mechanism, capable of carrying more channels in a given range of frequencies, cannot benefit therefrom if the original sources of data do not support the narrower bandwidths.
Similarly, a modern transmitting device cannot interface with legacy receiving equipment if the receiving device cannot provide the precision to distinguish signals within their comparatively narrow bands. Meanwhile, legacy equipment may be incompatible with carriers in that one component mismatched with another (e.g. in capacity), wastes the capacity of the underutilized element. Meanwhile, the great expense remains for upgrading each successive bottleneck in the transmission and receiving processes.
Thus, in general, having a mismatch of legacy equipment whether sending devices or receiving devices, in combination with either a modern narrow band sender or receiver, in view of the capacity of installed carrier media, results in either wasted capacity or expensive replacement of existing equipment.
What is needed is a mechanism for providing narrowing of bandwidth requirements. This would best be accomplished if such a device could xe2x80x9cdrop-inxe2x80x9d its modern, narrow-bandwidth capabilities within legacy networks.
The foregoing difficulties are overcome by data stabilization in accordance with the invention. In certain embodiments of an apparatus and method in accordance with the present invention. information may be transferred from one or more signals to an output signal that is easily phase locked with a carrier signal. Various photonic devices, including photonic transistors may be used to accomplish this end. Photonic amplifiers may provide amplification, preferentially in a single direction, suppressing amplification in an opposite direction.
Specific devices selected may rely on gas, dye, semiconductors, crystalline materials, or the like to provide the disparate amplification properties. For example, an amplifier having finite gain, when provided a continuous wave signal in one direction, will amplify the signal. A signal in the opposite direction, when its level reaches the reversing level of the device, loses energy from the process of amplification, causing reduced output.
Such a process provides an inverting function having a comparatively wide, frequency band pass for a modulated input, while transferring information in an inverted form to the frequency of a continuous wave bias signal. Since the signal is available for use by other local photonic circuitry, the output may be phase locked to the external photonic circuitry.
Applications for such an apparatus may include interfacing optical signals, such as those in the fiber of a legacy transmission system, in order to match to localized photonic circuitry in a transmitter or receiver. Provisioning and other processes that require allocation of frequencies may benefit from the transfer of information from one wavelength to another. Accordingly, a wavelength-division-multiplexing system may be operated more efficiently. Such a mechanism may operate for routing and controlling the signals to and from photonic devices.
One may think of a reversing level as a threshold function having multiple uses. For example, multiple inputs may sum to exceed the threshold in order to provide a multiple-input, multiple-frequency, multiple-phase logical AND device. Such a device provides a standardized output frequency. Multiple inputs, each having an intensity above such a threshold may provide a multiple-input, multiple-frequency, multiple-phased logical OR device.
In certain alternative embodiments, an amplifier may be part of a ring resonator or ring laser. The threshold function may be enhanced or modified by the lasing action existing within the ring resonator.
In one embodiment of an apparatus and method in accordance with the invention a silicon optical amplifier (SOA) may be used in a way dissimilar to it""s design performance. For example, the SOA may receive a single line laser output at a wavelength selected by a user. A control beam may be used to modulate the SOA with another laser.
The refractive index of the original SOA is changed by the laser source being modulated to embody data. The change in refractive index alters the gain of the SOA. Thus, the output of the SOA is inverted, and the gain will change with the data rate of the original source. A continuous wave reference laser used in such an arrangement may benefit by changing the bias point of the SOA. Some gain may be degraded, but the base band may be cleaned up somewhat. Also, since the data rate is governed by the gain, high data rates increase the gain and the SOA.
In one embodiment of a method and apparatus in accordance with the invention, modulated data from a photonic source within an initial transmission band may be modulated onto another photonic source having a different characteristic wavelength. One way to accomplish the effect is to rely on dual, optical, cross-modulation, utilizing some active media. For example, an SOA may serve well in this application.
Data modulated onto an initial photonic source may be passed, by way of a circulator into an active medium. The active medium, such as an SOA, may receive, in an opposite direction, a carrier signal from another photonic source (e.g. laser). The carrier signal from the second photonic source is modulated in the active medium, transferring the data from the original photonic source, onto which the data was modulated, into the new laser carrier at a different characteristic wavelength.
The newly modulated photonic signal (modulated carrier) may then pass through two circulators to an optical filter. The filter process suppresses residual light from the original photonic source of data. The output of the circulator to which the filter returns it""s output contains all of the data originally provided, but not modulated onto the laser carrier frequency of the new photonic source. Due to the SOA operation, the new or final output is inverted with respect to the original photonic data source. Various processes, including replication of the cross-modulation process just described may be used to restore the original signal.
In one embodiment, a signal from a legacy photonic data source may pass by way of a circulator through an SOA. Meanwhile, a signal from a reference photonic source (e.g. carrier, continuous wave laser, etc.) May pass through the SOA in an opposite direction. Data is cross-modulated onto the signal from the reference photonic source. The reference source signal having the data modulated onto it, passes by way of two circulators to an optical filter in order to attenuate or otherwise reject residual light from the legacy data source.
This process may be repeated with additional reference lasers, additional pairs of circulators, a corresponding SOA, and a corresponding filter. Accordingly, the output signal may be transmitted to a receiver remote therefrom, having been re-inverted by the second referential source and SOA.
In yet another alternative embodiment, a wavelength conversion may be executed by a transmitter device or system, being followed by a second conversion accomplished at a remote receiver. Such a process may provide a certain degree of encryption, as well as additional data channels by virtue of inversion during transmission.
In certain embodiments, a method and apparatus in accordance with the invention may provide repeatability of phase, frequency, or both relationships between an output of a photonic source and a reference source after one or both are shut down and restarted. Stabilization of phase and frequency relationships are important, but may be difficult.
In one embodiment, phase, frequency or both relationships between an electromagnetic oscillator (e.g. laser, etc.) and an outside system of photonic circuitry may be maintained, although the oscillator is off. Moreover, a method and apparatus in accordance with the invention may reestablish this same frequency and phase relationships once the oscillator portion starts up again.
A comparatively modest level of energy from a seed reference signal may be directed into an amplifier of an oscillator. When the oscillator is energized or modulated into an xe2x80x9conxe2x80x9d state, the amplifier adds energy to the existing phase established by the seed signal. As amplification continues, the oscillator becomes fully energized. During the rise time, the additional energy becomes tuned to the frequency and phase of the seed reference. Accordingly, when full power is achieved, the signal is xe2x80x9csynchronizedxe2x80x9d in phase and frequency with the seed reference signal. Regardless of the method of energizing the oscillator, whether optically, electronically, mechanically, or switched, the seeding process succeeds.
In one embodiment of an apparatus and method in accordance with the invention, a tamed spectrum multiplexing process may be executed in order to facilitate multiplexing of a legacy light source having an original wavelength band. Such sources (e.g. Fabry Perot laser systems) typically exhibit multi-mode, wideband, time-variant, spectral characteristics detrimental to multiplexing.
Semiconductor laser diodes exhibiting multi-mode behavior are not considered suitable for transmission applications requiring extended distances, nor for applications requiring multiplexing. Undesirable properties of the mode behavior typical semiconductor lasers, results in broad spectral signals, mode hopping, and so forth.
In an apparatus in accordance with the invention, hopping is suppressed while dispersion is decreased, increasing the range of transmission. Moreover, higher numbers of channels may be multiplexed together, due to the narrowed bandwidth requirements of each corresponding signal.
In one embodiment, feedback to a remotely located legacy device providing modulated data may provide a single mode photonic signal (e.g. light). Excitation of the legacy photonic source effectively collapses the output spectrum thereof into a signal mode near or at the frequency at the excitation source (seed). Accordingly, various benefits are provided. For example, cross bar switching, and thus, remote provisioning, becomes tractable. A simple interchange of the carrier frequencies of filters tunable in accordance with a multiplexing scheme at the receiver end facilitates the process.
In one embodiment of an apparatus and method in accordance with the invention, an active medium, such as an SOA, may provide a reference photonic source. The reference signal may be fed to legacy photonic sources originating a modulated photonic signal. Accordingly, certain spectral components may be substantially exclusively generated and relied upon for transmission of data.
For example, a selected region of the spectrum may be provided, having a substantially narrower bandwidth than the transmitting or receiving bandwidth of a legacy photonic device. Spontaneous emission from the SOA is transmitted to a filter, such as a grating or a reflecting Bragg filter. The reflective portion of the signal passed through the SOA causes an amplification of the selected wavelength reflected from the filter.
Meanwhile, the passband signal goes on to some place elsewhere. The result is a suppression of the spontaneous initial frequencies not consistent with the reflection band of the filter.
The output of the filtered signal, after passing back through the SOA, may return to a circulator intermediate the legacy photonic source and the SOA. Accordingly, the legacy photonic source is stimulated or seeded at the selected wavelength. The output of the legacy photonic source is ultimately provided as an output of the circulatory. Because the filtered signal is so narrowed, and amplified by the SOA during the return pass, power levels may be substantial to stimulate the legacy photonic source.
Broadband tuneability in lasers is difficult and expensive, if possible at all. Typically, complex dye lasers must be relied upon for such mechanisms. Such a massive physical plant is hardly suitable for integration in small scale telecommunications devices. Thus, broadband sources are extremely difficult to come by. Meanwhile, narrowband filters, tunable over a broad range of operation are likewise extremely difficult to come by. In a method and apparatus in accordance with the invention, the presence of either one facilitates the ability to obtain the benefits of the other.